Right middle lobe is more prone to atelectasis because its bronchus has a narrow diameter, acute angle of take-off and a relatively long course

Causes of right middle lobe atelectasis include

Asthma

Cystic fibrosis

Bronchopulmonary dysplasia

Foreign body aspiration in the airway

Tumors

Primary endobronchial (i.e. bronchogenic carcinoma)

Extrinsic compression from adenopathy

Granulation tissue, as in TB

Bronchopneumonia

Imaging findings

Right middle lobe atelectasis is usually easier to recognize on the lateral view than the frontal view, where it may produce very subtle findings

Silhouetting of the right heart border on the frontal view by the adjacent un-aerated medial segment of the middle lobe

If the atelectatic middle lobe swings upward and anteriorly, it may produce a wedge-shaped density on the frontal view with its base at the heart

Depression of the minor fissure and elevation of the major fissure, especially well seen on the lateral view

On the lateral view, the atelectatic lobe forms a triangular density with its apex at the hilum and its base more peripheral in the lung

If there is a nodular density seen at the apex of the triangle on the lateral view, suspect a mass in the hilum producing the atelectasis

Elevation of the right hemidiaphragm may occur

Treatment

Chest physical therapy

Bronchoscopy

Right Middle Lobe Syndrome

Frequently occurs in children, with asthma a relatively common etiology

Mainly based on chronic or recurrent right middle lobe atelectasis, rather than acute etiology

Some authors believe the loss of volume is a primary inflammatory condition of the middle lobe aided by its isolation from the upper and lower lobe rather than caused by obstruction

Right Middle Lobe Atelectasis. Frontal radiograph of the chest (left) demonstrates indistinctness of the right heart border (white arrow) and slight elevation of the right hemidiaphragm from volume loss. The lateral view of the chest (right) shows downward displacement of the minor fissure (red arrow) and slight upward bowing of the major fissure (black arrow), both bordering the increased triangular density of the atelectatic middle lobe.
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